Experimental investigation of mixing enhancement in a small mixing layer facility at low Reynolds number.

摘要

Many chemical and biological analyses are carried out in small devices which require rapid mixing of reactants. These miniaturized devices involve low speed flow wherein mixing occurs in the absence of turbulence. Microfluidic mixing devices are known to be poor mixers since molecular diffusion at the interface of flow streams forms the only mechanism of mixing. The key to enhance the mixing in such a low speed regime is to manipulate the contact area of two initially segregated flow streams.;In regards to the above mentioned problem, an experimental investigation is performed to understand the mixing field in a low Reynolds number forced wake flow. The flow velocity is so low that, in the absence of the imposed perturbation, mixing is primarily governed by interfacial diffusion mechanism which is similar to the situations commonly seen in microfluidic applications. To enhance the mixing interfacial area, flow perturbation is provided over a range of frequencies and amplitudes. The chemically reacting laser induced fluorescence technique (LIF) is used to quantify the level of mixedness, while single component molecular tagging velocimetry (MTV) technique is used to measure the amplitude of perturbation velocity. A non-reacting LIF technique is utilized for few selected forcing cases to study the distribution of mixed fluid composition. Each forcing frequency creates a unique interfacial mixed fluid structure with different levels of chemical product. For low forcing frequencies, mixed and unmixed fluid regions were noticed wherein mixed fluid was found on the interfacial structures. A large amount of chemical product was observed for certain high forcing frequencies which also corresponded to the highest perturbation velocity amplitudes, highlighting the large velocity dynamics involved in these cases. These highly mixed cases are also found to show asymmetric mixing characteristics. A chemically reacting LIF was also performed at lower forcing amplitude over all the forcing frequencies, and the amount of mixedness is seen to be directly connected to the forcing amplitude.;In addition to quantifying the mixedness, a stationary and reverse flow behavior of the mixed fluid structure was observed in certain forced cases. This phenomenon is discussed in detail by utilizing phase resolved streamwise chemically reacting LIF concentration fields.